Abstract
Suweon 258, Akenohoshi, Saikai 180, and Saikai 183 of Japonica-Indica crossbreds, and Tsukushihomare of Japonica rice cultivars were planted at the experimental field of Kyushu National Agricultural Experiment Station. The solution containing tracer NH_4-^<15>N at a small amount and high atom % (0.3 g/cm^2, 99.7 atom % ^<15>N) was randomly injected with a long thin needle into the plow layer of the ^<15>N experimental plot encircled with an iron frame of 30×15 cm^2. Twenty-four hours after the injection, rice plants were taken out and divided into root and shoot. From these determinations of ^<15>N atom %, the activity and vitality of the rice root tissue were investigated. The results obtained are summarized as follows : 1) There was an adequate correlation between the rate of ^<15>N absorption by rice plant and ^<15>N atom % excess of the root tissue. From this result, it was considered that ^<15>N atom % of the root tissue was a measure of the activity of root tissue when the vitality of root tissue was defined by the rate of nitrogen absorbed. 2) A large difference was recognized in the atom % excess of ^<15>N in the root tissue among these cultivars at each stage of the ripening period. It weakened with time. The amount in Tsukushihomare was the highest. The amount in Suweon 258 was the same as that in Tsukushihomare from heading to milky stage. But it was lower than the former from the dough stage. The amount in Akenohoshi was the same as that in Suweon 258. The amount in Saikai 180 and Saikai 183 were lower than that in Akenohoshi. 3) The vitality of root tissue at the milky stage was considered to be Suweon 258 = Tsukushihomare ≫ Saikai 180 ≒ Saikai 183 ≒ Akenohoshi. However, the amount at the dough stage was Tsukushihomare ≫ Suweon 258 ≒ Saikai 180 ≒ Akenohoshi.
